Device for monitoring the leakage current of a surge arrester

ABSTRACT

A device for monitoring a leakage current of a surge arrester has a measuring element for detecting a leakage current in a leakage current circuit. A filter assembly filters the leakage current in order to extract a third harmonic oscillation which is evaluated by an evaluation circuit with respect to the value thereof. The evaluation circuit delivers at least one warning signal to a display device. In a preferred embodiment, three display devices operating as a signaling light are provided. A power supply for the used circuits is obtainable by way of a transformer which extracts a necessary auxiliary zero-potential energy contained in the leakage current and with the aid of a main power supply unit connected to the transformer.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to a device for monitoring the leakage current ofa surge arrester in accordance with the precharacterizing clause of themain claim.

Surge arresters for electrical power transmission systems are extremelyreliable devices. However, it is often nevertheless desirable to monitorthe operating state of the arrester. This is particularly the case whenthe devices are subject to unusually stringent requirements, for exampleowing to the design or environmental conditions, or are installed atparticularly significant points in the system, such as at machinetransformers or strategically important substations. In this case, theuser would wish to be able to use monitoring devices to identify in goodtime whether the arrester is capable of reliably arresting surges havingthe specified power content at any time or whether the arrester has beendamaged or is at risk of failure. In addition to the established methodof introducing a monitoring spark gap in series with the arrester, inthe case of which witness marks on the polished electrodes of the sparkgap are evaluated, and in addition to the method of using surge counterswhich register the occurrence of an arresting process above a specificamplitude, methods involving the measurement of leakage currents areknown in particular in the case of modern metal-oxide arresters. In thecase of metal-oxide arresters, the active part is not DC-isolated fromthe power supply system but is connected permanently to the voltage. Theelectrical properties of the active part are thus reflected at any pointin time in the leakage current through the arrester. So-called totalleakage current measurement is known, in the case of which the peakvalue for the total leakage current is usually evaluated, use being madeof the fact that an increasing resistive component is superimposed onthe normally predominantly capacitive leakage current as the temperatureof the active part increases or as the voltage applied to said activepart increases, and this increasing resistive component increases thetotal leakage current. Various influencing variables which are includedin the total leakage current may, however, result in erroneousinterpretations, and only a low degree of information quality can beachieved with this method.

The Patent Abstract of Japan JP 11307226 A has disclosed a monitoringdevice for a surge arrester. The monitoring device is supplied from anexternal power supply device. The voltage produced by this externalpower supply device is synchronized with the system voltage of the surgearrester in order to ensure monitoring of the surge arrester.

Furthermore, the Patent Abstract of Japan JP 2003037932 A has discloseda surge arrester which has a device which monitors the ageing of thearrester. Leakage-current monitoring detects the leakage current of thesurge arrester. In addition, the number of arresting processes of thesurge arrester is monitored. When a predetermined number of arrestingprocesses has been reached, correspondingly differently colored lightsignals are output.

Patent Abstracts of Japan JP 09145759 A and JP 2000321318 A have eachdisclosed apparatuses for monitoring a surge arrester. For monitoringpurposes, the leakage current of an arrester is monitored and evaluatedin a computer unit. For this purpose, a Fourier transformation iscarried out, for example. The result of the analysis is represented ineach case on a display device.

The Patent Abstract of Japan JP 03001476 A has disclosed a monitoringdevice for inspecting a surge arrester. An instrument transformer isinserted into the ground current path of a surge arrester. In this case,the ground connection forms the primary winding. The instrumenttransformer is fed an inspection current from an external AC voltagesource.

The Patent Abstract of Japan JP 08017552 A has disclosed a device formonitoring a leakage current of a surge arrester. The leakage current ofthe surge arrester is monitored by means of a detection device. Aresistive component is calculated from the measured leakage current bymeans of a computation unit. This resistive component of the leakagecurrent is used to diagnose the state of the surge arrester.

The zinc oxide material used, for example, for metal-oxide arresters hasa conductance which is nonlinear as a function of the voltage and leadsto the formation of a third harmonic component in the resistive leakagecurrent of the arrester when a sinusoidal voltage is applied. If theresistive component of the leakage current is increased, for example bymeans of degradation, this results in a shift to the range of alterednonlinearity of the characteristic and thus in a further rise in thethird harmonic component content. Methods are therefore known in whichanalysis of the third harmonic of the leakage current is carried out.For this purpose, the leakage current is generally output via ameasuring element and passed via a filter arrangement, by means of whichthe third harmonic component is filtered out and is evaluated in termsof its amplitude. However, at present this method has the disadvantagethat the available devices are costly and require a high degree ofexperience for correctly measuring and interpreting the measured values.In addition, these devices require auxiliary power for their voltagesupply and are therefore generally not used for continuous long-termmonitoring on arresters.

SUMMARY OF THE INVENTION

The invention is therefore based on the object of providing a device formonitoring the leakage current of a surge arrester using the thirdharmonic of the leakage current, in the case of which no auxiliary poweris required for the supply of the components and the measurement resultscan be evaluated in a simple manner, and which can be produced in acost-effective manner, with the result that continuous long-termmonitoring of the surge arrester is possible.

This object is achieved according to the invention by the characterizingfeatures of the main claim in conjunction with the features of theprecharacterizing clause.

As a result of the fact that the filter arrangement is connected to anevaluation circuit for the third harmonic, which, if appropriate,outputs at least one warning signal to a display apparatus which isconnected to the evaluation circuit, and the fact that a transformer isconnected in the arrester circuit, via which transformer the power for avoltage supply to the filter arrangement and the evaluation circuit canbe output, no auxiliary power is required for the electronic components,i.e. the latter are fed the leakage current, and no user-dependenterroneous interpretation is possible and only low demands are placed onthe qualifications of the user since the result of the monitoring isdisplayed. Furthermore, the device can be produced from standardelectronic components, as a result of which it is cost-effective. Thedevice can therefore be installed permanently at the arrester forcontinuous monitoring purposes.

The measures specified in the dependent claims make advantageousdevelopments and improvements possible. A measuring resistor canadvantageously be used as the measuring element, but the use of acurrent-compensated coil arrangement comprising a current compensator isalso possible.

An active bandpass filter having a frequency of 150 Hz can be used asthe filter arrangement, and a microprocessor can be used as theevaluation circuit. In one simplified embodiment, the microprocessor canbe replaced by a single discrete or integrated threshold value switch.

In one advantageous embodiment, the display apparatus has one or morelight-emitting diodes, in which case only one light-emitting diode canbe provided for the simplified version with the threshold value switch,which light-emitting diode displays the instance of a predeterminedthreshold value being exceeded. However, three light-emitting diodeswith the colors of a traffic light are particularly advantageouslyconnected to the microprocessor and display the fault-free, the criticaland the faulty operating modes. It is particularly advantageous toconnect a coil arrangement having a toroidal core and two windings,which are wound around the toroidal core and whose connections arepassed to the outside, into the arrester circuit, since, in the criticalcase, i.e. when the evaluation circuit outputs a warning signal, anexternal measuring device can be connected which can carry out a moreaccurate measurement and evaluation of the operating state.

Exemplary embodiments of the invention are illustrated in the drawingand will be explained in more detail in the description below. In thedrawing:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows one circuitry refinement of the monitoring device accordingto the invention,

FIG. 2 shows a further embodiment of the checking circuit used in FIG. 1for the third harmonic, and

FIG. 3 shows a further embodiment of the evaluation circuit used in FIG.1 with a display.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The device for monitoring the leakage current illustrated in FIG. 1 hasa metal-oxide arrester 1 having a protection spark gap 2 for arrestercurrents and the actual leakage current path 3. In addition to thearrester 1, the device comprises three switching units, a voltage supplyunit 4, a checking circuit 5 for the third harmonic and an evaluationunit 6. The checking circuit 5 has a measuring resistor 7, which isconnected into the leakage current path 3, and an active bandpass filter8 having a mid-frequency of 150 Hz. In addition, a coil arrangement 9 isarranged in the leakage circuit 3, said coil arrangement 9 having atoroidal core 10 and two coils 11 wound around the toroidal core, theconnections of the coils being passed to the outside for connection toan external measuring device.

The evaluation unit 6 essentially comprises a microprocessor circuit 12and three light-emitting diodes 13, 14, 15, which, in the exemplaryembodiment, are selected to be the colors of a traffic light, namelygreen, amber and red. Of course other light sources can in principlealso be used, in which case, if appropriate, one or more audible displayelements are also conceivable in place of the visual display elements.

The voltage supply unit 4 comprises a transformer 16, whose primarywinding 17 is connected into the leakage current path 3, and whosesecondary winding 18 is connected to a switched mode power supply 19having a rectification circuit and a filter circuit. The bandpass filter8 and the microprocessor circuit 12 are connected to the outputs of theswitched mode power supply 19 for their voltage supply. The auxiliarypower is output, floating, from the leakage current flowing in theleakage current path 3 via the transformer 16, the secondary coil 18providing the switched mode power supply 19 with an alternating current.The switched mode power supply carries out rectification and filteringand makes a positive and a negative DC voltage having a correspondingzero potential available at its output.

During operation of the arrester 1, the leakage current is checked bythe measuring resistor 7 and passed on to the active bandpass filter 8,which filters out the third harmonic from the checked leakage current.The microprocessor circuit 12 evaluates the third harmonic on the basisof the magnitude of its peak value and drives the LEDs 13 to 15. Forthis purpose, the microprocessor circuit 12 contains three thresholdvalues, in which case, in the event of a peak value below a firstthreshold value, normal operation is indicated by a green LED 13, in theevent of a peak value above the first threshold value but below thesecond threshold value, the critical range is indicated by an amber LED14, and, in the event of a peak value above the second threshold value,faulty operation is indicated by a red LED.

As has already been explained above, the coil arrangement 9, which canbe connected as a current-compensated coil, is used merely as anadditional sensor for an external measuring device for precise,compensated measurement of the third harmonic, if, for example, theamber LED 14 blinks.

FIG. 2 illustrates a further embodiment of a checking circuit 5, whichcan be used in place of the checking circuit shown in FIG. 1, i.e. themeasuring resistor 7 shown in FIG. 1 is replaced by acurrent-compensated coil arrangement, which has a toroidal core 20having two coils 21, 22 and a current compensator 23 connected to theconnections of the coils 21, 22. In this case, the total magnetic fieldof the coil arrangement is brought to zero by means of a reversecurrent, with the result that the measured voltage is available at theoutput of the current compensator 23. The active bandpass filter 8 inturn filters out the third harmonic.

FIG. 3 illustrates one variant of the evaluation unit 6, in the case ofwhich the microprocessor circuit 12 is replaced by a single thresholdvalue switch 24, which has only one threshold value for the display ofthe critical operating range. In this exemplary embodiment, there isonly one LED 25, which can illuminate or blink amber, for example.

The coil arrangement 9 can also be provided in all of the exemplaryembodiments.

1. A device for monitoring a leakage current of a surge arrester,comprising: a measuring element for checking the leakage current flowingin an arrester circuit connected to the surge arrester; a filterarrangement for filtering out a third harmonic from the leakage current,the third harmonic being a measure of an operating state of the surgearrester; an evaluation circuit for the third harmonic connected to saidfilter arrangement; a display apparatus connected to said evaluationcircuit for outputting, if appropriate, at least one warning signal; anda transformer connected in the arrester circuit, said transformer havingan output carrying an energy for a voltage supply to said filterarrangement and said evaluation circuit.
 2. The device according toclaim 1, wherein said transformer has a primary winding connected in thearrester circuit, and a secondary winding connected to a rectifiercircuit, and said rectifier circuit is connected to said filterarrangement and said evaluation circuit.
 3. The device according toclaim 1, wherein said measuring element is a measuring resistor.
 4. Thedevice according to claim 1, wherein said measuring element is acurrent-compensated coil assembly with a current compensator connectedto said filter arrangement and a rectifier circuit.
 5. The deviceaccording to claim 1, wherein said filter arrangement comprises anactive bandpass filter.
 6. The device according to claim 1, wherein saidevaluation circuit comprises a microprocessor circuit configured toevaluate peak values of the third harmonic and to output at least onewarning signal.
 7. The device according to claim 1, wherein saidevaluation circuit is a threshold value circuit configured to output awarning signal when a threshold value is exceeded.
 8. The deviceaccording to claim 1, wherein said display apparatus includes a visualdisplay.
 9. The device according to claim 8, wherein said displayapparatus has at least one LED.
 10. The device according to claim 1,wherein said display apparatus is configured to output an audiblewarning signal.
 11. The device according to claim 1, wherein saiddisplay apparatus comprises a plurality of LEDs, and said evaluationcircuit is configured to drive different LEDs based on a magnitude ofrespective peak values.
 12. The device according to claim 1, whichcomprises a coil assembly with a toroidal core and two coils woundaround said toroidal core connected in the arrester circuit, said coilshaving connecting terminals for connection to an external measuringdevice on occasion of a presence of a warning signal.